Transmitter noise can interfere with communication systems, such as public safety radio communication systems used in a mobile environment. Traditionally, filters have been used in the transmitter line-ups of such systems to reduce noise in the receiver band. Noise in the receiver band having an impact on transmitter performance can generally be referred to as far out noise. High power transmitters require filters that provide high rejection capability to reject far out noise. These filters can be costly and large in size making them undesirable in a production environment. While duplexers could be used to address the far out noise issue, in devices such as high power data (HPD) multi-band radios (35 Watts, 700/800 MHz), any duplexer that could handle the power and stringent noise specifications would be even larger and more costly than a filter.
FIG. 1 shows a traditional transmitter line-up 100 for a high power device including three gain stages 102, 104, 106 having a single filter 108 placed between two of the stages, here shown as stages 2 and 3. This filtering approach imposes restrictions on how the gain is distributed in the transmitter line-up as well as restrictions on the filter itself. For example, if there is too much gain before the filter 108, then the filter rejection will be insufficient to reduce noise in the receiver band. If there is too little gain before the filter 108, the transmitter will not meet power requirements. For the numerical examples provided in transmitter line-up 100, the amplifiers 102, 104 will amplify both the desired signal and the input noise by 50 dB before getting to the filter 108. Thus, filter 108 must also be able to handle high power. For a high performance data product, the filter ends up being extremely large, difficult to manufacture, costly and faces significant issues with placement in the assembly process.
Accordingly, it would be beneficial to have an improved transmitter line-up capable of handling far out noise.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.